1. Technical Field
The present disclosure relates to semiconductor devices, image sensors, and methods of manufacturing the same and, more particularly, to semiconductor devices having a trench, CMOS image sensors having a trench, and methods of manufacturing semiconductor devices and the CMOS image sensors.
2. Discussion of the Related Art
Image sensors are semiconductor devices that convert optical images into electrical signals. Image sensors can be classified into Charge Coupled Device (CCD) image sensors and CMOS Image Sensors. A CMOS image sensor includes a photodiode that receives optical signals and a MOS transistor that controls the optical signals within a unit pixel. The CCD image sensor has a complicated driving system and a complicated manufacturing process. Signal processing circuits are difficult to fabricate in one CCD chip. In contrast, the CMOS image sensor can be manufactured by standard CMOS techniques, and can be integrated into a single circuit together with other signal processing circuits.
CMOS image sensor fabrication includes forming a device isolating layer on a silicon substrate to define the photodiode active regions and MOS transistor active regions. A CMOS image sensor may exhibit crystalline defects such as dangling bond at an interfacial surface between the device isolating layer and the substrate of the photodiode active region. For example, when the device isolating layer has a Shallow Trench Isolation (STI) structure, such as a trench formed by etching the substrate and filled with an insulating layer, crystalline defects may occur when etching the substrate. The crystalline defects, which act as traps capturing electrons, may become defect or noise components of each pixel, increasing the dark current, i.e., the current that continues to flow in the photodiode when there is no incident light. Thus the crystalline defects of the device isolation region can degrade the imaging characteristics of the CMOS image sensor.
Forming an impurity region within a lower portion of the trench using ion implantation is difficult. Because the ion beam used for ion implantation has forward directivity, the impurity region is mostly formed under the trench without surrounding the sides of the trench. The ion beam or substrate may be tilted during the implantation process so that the impurity region is formed on the sides of the trench, but with no impurity region formed under the trench. A crystalline defect occurring in the interfacial surface of a trench that is not surrounded by an impurity region may generate the dark current. As semiconductor devices become ever more highly integrated, the trenches are further deepened and narrowed. In such case, it becomes more difficult to form the impurity region by ion implantation to surround the trench. As a result, the dark current may be increased.
Moreover, because ion implantation is performed under a relatively high energy condition, the impurity region is generally thickly formed. This is apt to shrink the depletion region of the photodiode formed adjacent to the trench, which may decrease the saturation current of the photodiode.